Bugfix: Restore ARMv5 support for ARM binaries

The official release binaries for restic 0.16.1 were accidentally built to
require ARMv7. The build process is now updated to restore support for ARMv5.

Please note that restic 0.17.0 will drop support for ARMv5 and require at least
ARMv6.

https://github.com/restic/restic/issues/4540
